1. Technical Field
The present invention relates to a spring, and more particularly relates to a spring which uses a fluid, such as electrorheological fluid or magnetic fluid, to provide a variable spring rate and a variable load-carrying capacity.
2. Background Art
Devices using electrorheological fluid or magnetic fluid to control an operating characteristic of the device are known. The resistance to flow of the fluid in such a device varies as a function of the magnitude of either an electric field or a magnetic field applied to the fluid. One device using electrorheological fluid to provide control of an operating characteristic of the device is disclosed in U.S. Pat. No. 4,790,522, issued on Dec. 13, 1988 and assigned to the assignee of the present invention. U.S. Pat. No. 4,790,522 discloses a shock absorber and a fluid spring used in a vehicle suspension system. A control valve controls flow of a hydraulic fluid between two fluid chamber portions of the shock absorber to vary the operating characteristics of the shock absorber and the fluid spring. An electrorheological fluid resists movement of the control valve to the open position. The control valve moves to an open position when the fluid pressure in one of the fluid chamber portions acts on the control valve to overcome the resistance of the electrorheological fluid.
Another known device using electrorheological fluid to provide control of an operating characteristic of the device is disclosed in British Patent Application No. 2,111,171. The device disclosed in British Patent Application No. 2,111,171 is a shock absorber having fluid chamber portions filled with electrorheological fluid. The fluid chamber portions are connected in fluid communication with each other through fluid passages. The amount of fluid flow through each fluid passage varies as a function of the magnitude of an electric field applied to the fluid in the fluid passage. The amount of fluid flow through the fluid passage decreases when the resistance to flow of the electrorheological fluid in the fluid passage increases. Likewise, the amount of fluid flow through the fluid passage increases when the resistance to flow of the electrorheological fluid in the fluid passage decreases. The damping characteristics of the shock absorber are varied when the amount of fluid flow through each of the fluid passages is varied.
A known damping device using magnetic fluid to control operating characteristic of the device is disclosed in U.S. Pat. No. 3,059,915. The device of U.S. Pat. No. 3,059,915 controls flow of a magnetic fluid through a fluid passage. The amount of fluid flow through the fluid passage varies as a function of the magnitude of a magnetic field applied to the fluid in the fluid passage. The damping characteristics of the device depend upon the amount of fluid flow through the fluid passage.